1. Field of the Invention
The present invention relates to an obliquely deposited film element having an obliquely deposited layer which is formed on a substrate, especially used for a polarization element or the like, an important constituent in optical communication, photo detection, light processing, and the like.
2. Description of the Related Art
Heretofore, in the field of optical communication, photo detection, light processing, and the like, control of light phase has been necessary. Therefore, phase control elements of various kinds have been used. One of these phase control elements is an obliquely deposited film element in which an obliquely deposited film is used.
The obliquely deposited film comprises columns with a thickness of approximately 10 nm, which aggregate at an angle inclined to the direction of an deposition source from the normal of the substrate surface and form an anisotropic fine structure inside the plane of the film. Due to such an anisotropic dense structure, various physical properties inside the plane of the film have anisotropy.
For example, an obliquely deposited film made of Ta.sub.2 O.sub.5, a substance which is transparent to visible light, has a double refraction characteristic to vertical incident light. Therefore, this obliquely deposited film is used for a phase plate in a sphere of visible light.
An obliquely deposited film often has a problem of relaxation of a columnar structure, in other words, coagulation in each of columns which constitute the aforementioned dense structure. Relaxation of columnar structure causes scattering of light in the film, thereby clouding the film. Therefore, the characteristic of the film changes and quality of an optical element, such as a phase plate, deteriorates.
Methods for avoiding such clouding to some extent are known. For example, it is known that by removing contaminants on a substrate to the utmost and performing high speed oblique deposition of the Ta.sub.2 O.sub.5 in a high vacuum, relaxation of the columnar structure in the obliquely deposited film is diminished and a less cloudy film element under visible light is produced.
However, with regard to an obliquely deposited film element produced in the manner mentioned above, sufficient durability has yet to be obtained under severe circumstances, such as high temperature or high humidity.
In several fields which require utilization of light, it is expected that a wavelength of light is shortened for the purposes of improving resolution or the like. However, Ta.sub.2 O.sub.5, which is the material for the film of the aforementioned obliquely deposited film element, absorbs light with a wavelength of 450 nm or less. Therefore, the Ta.sub.2 O.sub.5 cannot be used for a phase plate which is applied to ultraviolet light.
On the other hand, a method for avoiding the clouding of an obliquely deposited film including a substance transparent to ultraviolet light, such as SiO.sub.2, is not yet known.
The present invention is directed to providing an obliquely deposited film element which prevents the relaxation and clouding of the columnar structure of an obliquely deposited film and scarcely relies on the substance used as a film material.